Oil and gas companies are devoting more and more resources to searching and exploring deep waters (e.g., depths of 5000 feet or more). However, deepwater exploration is an expensive undertaking due to the difficulty of operating in deepwater environments. The higher costs have led to curtailment of certain types of reservoir evaluations, such as open hole density and neutron porosity logs, in some deepwater environments. On the other hand, cased hole neutron porosity logs are increasingly being acquired in such environments because operational risks are smaller in cased holes. These neutron porosity logs can be used to supplement open hole logging programs in some deepwater environments.
One challenge facing deepwater cased hole logging operations is the casings in deepwater wells need to be thicker compared to wells in shallow waters and on land. As a result, corrections for downhole conditions that are normally applied to shallow-water and on-land neutron porosity logs may not be applicable to neutron porosity logs recorded in deepwater cased holes. The corrections are commonly plotted as individual correction charts compiled in chartbooks that were developed over many years for neutron porosity logs in both open and cased holes. For cased hole neutron porosity logs, the chartbooks typically include corrections that account for the influences that casing thickness, cement thickness, and similar environmental parameters may have on logging responses. These corrections are intended to correct logging responses produced under logging conditions that are deemed to be nonstandard to those produced under standard logging conditions.
In the chartbooks, corrections that account for cement thickness have been expanded over the years to include the effects various oil field cement mixtures can have on the cement thickness correction. However, little has been done in the industry to extend casing thickness corrections beyond approximately one-half inch, which is well short of the casing thicknesses encountered in deepwater wells. In addition, both types of corrections have typically been applied under the assumption that they are independent of other environmental parameters. That assumption, however, may not be valid in the case of casing thickness corrections, especially in deepwater wells where thicker casings and cement sheaths are common.
Presently available solutions address the deepwater correction problem by using Monte Carlo simulations to simulate an array of log responses for several combinations of formation porosity and casing thicknesses. These simulations are typically carried out using a casing inner diameter that matches the casing inner diameter under standard logging conditions. The simulations then simulate log responses for a plurality of casing thicknesses by changing the thickness of the casing while increasing the diameter of the borehole to maintain a cement thickness that corresponds to the standard cement thickness (about 1.25 inches). A correction algorithm is thereafter derived based on the simulated log responses and subsequently used to account for the thicker casings of deepwater wells.
A drawback of the above approach is, if the influence or effect that casing thickness may have on logging responses is in fact coupled with another parameter, such as borehole diameter, then the casing thickness correction, derived as described above, may not be accurate. In that case, applying the casing thickness correction may produce inaccurate results because the simulations would include the coupling, but the correction would not. An alternative approach is to simulate multiple arrays of log responses for numerous combinations and permutations of porosity and environmental parameters that depart from standard logging conditions. However, the complexity of such a multi-array simulation would be extremely difficult and expensive.
Accordingly, a need exists for an improved way to correct cased hole neutron porosity logs for casing thickness effects, especially in deepwater wells where thicker casings are common.